The present invention relates generally to web-forming sections of a paper machine and, more particularly, to a web-forming section intended to modernize or constituting a modernization of a fourdrinier wire part.
Specifically, the present invention is directed to a web-forming section intended for the modernization of a fourdrinier wire part and which comprises a fourdrinier wire loop situated in respect to a headbox of the paper machine to form a single-wire, substantially horizontal initial portion of the dewatering zone in which the stock from which the web is to be formed is dewatered through a lower wire by means of draining means, the web-forming section also comprising an upper-wire unit including an upper wire guided by guide and web-forming rolls, the upper wire together with the run of the lower wire forming a two-wire dewatering zone within which dewatering takes place substantially upwardly through the upper wire.
Fillers, normally constituted by mineral substances, are often incorporated within paper in order to improve the printing and technical properties of the paper. The fillers are usually added into the pulp stock in the form of a water sludge. However, fillers do not adhere well to the fiber network of the stock which is a main reason for the poor retention of fillers therein. For this reason the filtering effect of the fiber network withholding the filler particles becomes an important factor affecting the retention of the fillers. The degree of filtering effect provided by the fiber network is determined by the thickness of the pulp web running on the wire, by the density of the fiber network, by the density of the wire and, moreover, by the draining or dewatering effects applied to the web.
Like the fines in paper, such as flours and coloring agents, fillers tend to be unevenly distributed in the direction of the thickness of the paper thereby causing a so-called unequal-sidedness in the paper. The unequal-sidedness of paper manufactured in fourdrinier machines results from the fillers being washed away along with the water which is drained from the lower portion of the pulp web in the filtrate water whereby the top portion of the web is enriched in fillers relative to the bottom portion of the web.
As is well known, attempts have been made to reduce the problems of unequal-sidedness of paper, not only by means of additives improving the retention of the fillers in the fiber network, but also by means of providing for a gentle dewatering during the initial filtering stage.
In prior art twin-wire formers, or so-called full-gap formers, which are now in common use, the pulp is supplied onto the wire part as a thin sludge whereupon a violent dewatering of the pulp web is begun immediately or after a short single-wire section, in both directions or in the same direction as in a single-wire section. This results in a considerable quantity of the fillers which have been added to the pulp, e.g. bolus, as well as fine fibers being carried away from the web along with the water being drained therefrom. Of course, this results in a considerable deterioration in the quality of the paper and, in particular, impairs the very properties intended to be provided to the paper by means of the fillers. Moreover, a simultaneous and violent two-sided dewatering also results in a weakening of the mid-portion of the paper web which in turn results in a low internal bond strength.
A two-wire former is disclosed in Finnish Pat. No. 50,648, assigned to applicants' assignee, in which the drawbacks discussed above are attempted to be avoided. This two-wire former is characterized by a single-wire initial portion of the wire part which is sufficiently long so that while a gentle dewatering takes place in the initial portion, the pulp web has time to obtain such a degree of felting prior to a two-wire portion of the wire part that the fibers can no longer be significantly shifted with respect to each other. Moreover, the two-wire portion of the wire part is guided, such as by a draining roll or by a draining box, so as to be curved downwardly whereby water is drained in the curved portion through the upper wire in a direction opposite to the direction of dewatering in the single-wire initial portion by the effects of centrifugal force and of a pressure zone produced by the tensioning between the wires. The main objective is to reduce the removal of additives to the pulp web, such as fillers, as well as to reduce the removal of fines from the pulp web and to increase the internal bond strength of the paper being manufactured.
It is well-known that in a conventional fourdrinier machine, dewatering of the web takes place only in the downward direction so that fines and filler agents are removed from the side of the web which faces the wire due to the washing effect of the foils or table rolls. For this reason a web manufactured in such a fourdrinier machine is anisotropic in regard to the properties of its two sides, the upper side of the web being smoother and containing more fines and fillers than the wire side. Moreover, the wire side of the web is left with a mark from the wires.
For the above reasons, paper made by means of two-wire formers is considered superior, especially with respect to printing properties. In such prior art two-wire formers in which no stationary dewatering elements are utilized, formation is usually poor and no pulsations of the dewatering pressure can be produced which would improve the formation. Another drawback of such prior art formers is that the same are not capable of adjusting the ratio of the quantities of water being dewatered through the upper and the lower wires. The desirability of providing the capability for such an adjustment has been expressed on several occasions.
Two-wire formers are also known in the art wherein the dewatering is mainly effected by stationary dewatering elements. However, in such prior art two-wire formers a drawback is present in that filler and fine retention is relatively poor whereas wire wear and power consumption is high.
Recently, modernizations of fourdrinier machines have become common in which one or more upper-wire units are situated above the fourdrinier wire unit by means of which an upward dewatering of the web is achieved with the objective of both increasing the dewatering capacity as well as improving web formation and filler and fine retention. An increased dewatering capacity in turn permits an increase in the speed of the paper machine. A further aim of such modernized fourdrinier machines is to provide the capability of reducing the thickness of the pulp supplied from the headbox which itself is advantageous. In certain cases, old low-speed news print machines have been converted or modernized by means of the upper-wire units of the type described above into board machines without increasing the speed of the machine. It is for these modernizations that the present invention provides an advantageous improvement.
As examples of prior art arrangements for modernizing fourdrinier wire parts, reference is made to Finnish patent application No. 782,709 (Beloit Walmsley Ltd.) and to British Pat. No. 1,582,342 (Australian Manufacturers Ltd. and Beloit Walmsley Ltd.). Reference is also made to U.S. Pat. No. 4,154,645 and to Finnish patent applications Nos. 810,373 and 811,514, all assigned to applicants' assignee.